Astigmatism discussed in this invention disclosure is an astigmatism formed at the optical axis when the optical system is not rotationally symmetric about the optical axis. This is usually due to a toroidal shape of at least one of the ocular surfaces of the eye, most commonly the anterior corneal surface. This type of ocular astigmatism is called corneal astigmatism. Toroidal shape is two-curvature shape described by a surface with meridians of steepest and flattest curvatures located at right angle to each other. These meridians and principle meridians. A difference between the curvatures in terms of dioptric power is also called cylinder power of the toric lens. The described type of ocular astigmatism is also called regular astigmatism which is correctable by an optical aid such as spectacles, contact lens, corneal implant or intraocular lens.
Ocular astigmatism may also be due to an ocular surface of the eye is transversely displaced or tilted, most commonly a surface of crystalline lens in phakic subjects. This type of astigmatism is called lenticular astigmatism. Lenticular astigmatism almost invariably manifests the flattest meridian close to vertical orientation and usually does not exceed 1.5 D cylinder. Corneal astigmatism on the other hand, manifests large variation in the meridian orientation and magnitude.
Astigmatism correction commonly involves correction for other ocular deficiencies such as myopia (nearsightedness), hyperopia (farsightedness), aphakic and presbyopia, and the corrective toric ophthalmic lens may include spherical corrective power and multifocal corrective power for the corresponding ocular deficiency correction.
The astigmatism correction requires proper toric lens orientation—the lens flattest meridian to be aligned with the steepest meridian of the cornea, or steepest meridian of the eye refraction error if lenticular astigmatism is involved. The toric lens alignment in reference to ocular astigmatism meridian, so called rotational alignment or meridional alignment, is the most critical factor of a toric lens performance requirement in order to allow maintaining an acceptable image quality.
Eye's astigmatism that combines corneal and lenticular astigmatism comes into play for toric contact lens or phakic IOL applications, i.e. the IOL that compliments natural crystalline lens. Corneal astigmatism comes into play for toric aphakic IOL application. i.e. the IOL that replaced natural crystalline lens. This disclosure will reference below to astigmatism of eye as a general reference that includes either eye's astigmatism or corneal astigmatism and a reference to a toric lens includes either a toric contact lens or phakic IOL or aphakic IOL.
Thus, a toric lens is designed to compensate for a cylinder of the eye in the aligned position of its cylinder meridian with the eye's cylinder meridian and the lens toric cylinder magnitude is the same as eye's cylinder magnitude but they are of opposite sign. Assuming that in this condition the eye does not manifest any refractive error, i.e. both cylinder and spherical error are corrected. A meridional misalignment of a toric lens leads to a resultant cylinder. A resultant cylinder magnitude is proportional to both: eye's cylinder intended to be corrected by the toric lens and a magnitude of meridional misalignment angle, i.e. angle between toric axes of the lens and eye. A spherical error correction is not impacted by a misalignment, meaning if a spherical error was corrected in the aligned toric lens position, a toric lens meridional misalignment does not introduce a spherical error and only effect resultant cylinder.
The attempt to reduce sensitivity to toric lens meridional misalignment can be found in U.S. Pat. No. 5,570,143 by Newman where aspheric surface shape that induced a depth of focus was discussed. The lens according to Newman's invention includes optical topography on the surfaces of the lens which induces a depth of focus. When a lens meridians line up with the meridians the cylinder power requirement is fulfilled. If the meridians do not line up, i.e. a condition of meridional misalignment, the author speculates that the depth of focus corrects for the resulted overrefracton. The Newman's patent references to 1.5 D depth of focus as an example.
The Newman's patent relies on the assumption that an increase in depth of focus (depth of field) increases the width of each tangential and sagittal focal segments thus improving their overlap at the best image defined by the circle of least confusion and, therefore, image quality for the corresponding meridional misalignment as compared with a toric lens without a depth of focus increase. Conceptually it is correct but there are two fundamental issues with the Newman's patent: (a) an increase in depth of focus reduces the image quality and no specificity has been provided in the patent in this regards, and (2) depending upon the corresponding asphericity design a depth of focus might increase the overlap between tangential and sagittal focal segments, keep the same or even reduce it.
Another attempt to address meridional misalignment can be found in U.S. Pat. No. 5,796,462 by Roffman. The Roffman's patent describes asphericity application to each toric meridian in a form of a prolate aspheric curve. Similar to Newman's disclosure, the patent relies on the increased depth of focus to reduce sensitivity to the meridional misalignment but also applying it to the toric surface in such a way that the effective cylinder decreases from the center of the lens towards the periphery of the optical zone. The issue with this approach is that the described aspherization leaves residual cylinder even with lens perfect meridional alignment. Similar to Newman's patent, the disclosure in based upon refraction scalar consideration in addressing meridional misalignment despite the fact that a system with a toric surface manifests vector characteristics. In addition, no disclosure of the particulars of the aspheric surface that increases a depth of focus with positive impact in the image quality has been disclosed.
In view of the prior arts limitations, there is a need for improvement of the toric ophthalmic lens design that reduces sensitivity to meridional misalignment.